Point defects, thermodynamic and superconducting properties of the non-stoichiometric HgBa2CuO4+delta phase

The thermodynamic properties of Hg1201 have been studied by thermogravimetry in terms of point defects, related to the departure from stoichiometry delta. The oxygen intercalation/disintercalation process is shown to be not completely reversible in the temperature range of 150-450 degrees C. The reversible fraction delta* depends on the oxygen pressure according to a law in (pO(2))(1/6) which corresponds to the insertion of interstitial oxygen and the formation of two holes per oxygen. The formation enthalpy of these defects, 0.5 eV, is rather small. A model based on temperature dependent barrier heights is introduced to account for the non-reversible fraction. The barrier mechanism may be related to exchange interactions between interstitial oxygens mediated by the mercury sublattice. The thermogravimetric analysis has allowed to derive the T-c versus delta curve without resorting in particular in the overdoped regime to high oxygen pressures.